1. Field of the Invention
The present invention relates to ratio controls for toroidal-type traction drives.
2. Description of the Background Art
Continuously variable transmissions (CVT""s) are transmissions that change a speed ratio continuously, not in discrete intervals. This continuous nature of CVT""s gives them an infinite number of speed ratios, making them very attractive for automotive use.
Various types of CVT are known. One such example is a CVT with pulley/V-belt power transfer. Another example is a CVT with disc/roller power transfer. The CVT of this type is often referred to as a toroidal-type CVT (T-CVT) because it transmits torque from one rotating semi-toroidal disc to another semi-toroidal disc by traction rollers through a traction force. The two semi-toroidal discs form a toroidal cavity. In each toroidal cavity, it is preferred to have two traction rollers in equiangularly spaced relationship engaging the discs for transmission of motion therebetween. While three or four traction rollers may be disposed in spaced relationship in each toroidal cavity and will provide increased life for contact surfaces as the total surface area is increased, two traction rollers are preferred for simplicity.
Traction roller are rotatably supported by pivot trunnions, respectively. The pivot trunnions, in turn, are supported to pivot about their respective pivot axis. In order to controllably pivot the pivot trunnions for a ratio change, a hydraulic control means is provided. The hydraulic control means is included in a hydraulic cylinder at each pivot trunnion and includes a control volume defined in the hydraulic cylinder between a piston and an axial end of the hydraulic cylinder. The pistons within the hydraulic cylinders are connected to the pivot trunnions along their pivot axis by rods. The piston and its associated rod are thereby rotatable about the pivot axis with the associated pivot trunnion. Variation of the control volume causes the piston to move relative to the hydraulic cylinder, and applies a control force to displace the pivot trunnions. Control forces applied displace the pivot trunnions in the opposite directions along their pivot axis. As a result, the pivot trunnions are caused to pivot about their respective pivot axis, due to the forces present in the rotating toroidal discs, for initiating ratio change.
For terminating the ratio change upon attainment of a desired ratio, a feedback structure or mechanism is provided. The feedback structure preferably includes a source of hydraulic pressure, and a ratio control valve for controlling the flow of hydraulic fluid for initiating ratio change. The feedback structure further includes a mechanism associated with at least one pivot trunnion to adjust the ratio control valve upon pivotal movement of the pivot trunnion to a desired ratio. The mechanism is preferably a feedback cam connected to a pivot trunnion. The cam may be linked mechanically and/or electronically to operate the ratio control valve upon reaching a desired rotation.
In most cases, a feedback cam is fixed to the bottom end of a predetermined traction roller assembly including a trunnion. The trunnion includes a backing plate with an extension about which a traction roller rotates so that it is subject to thrust load. Thrust load increases as input torque increases. To withstand such thrust load, the trunnion is supported at two spaced mount sites. At high input torque, the traction roller is urged inwardly into the traction cavity, pulling the trunnion inwardly at a portion between the two mount sites. In such stressed state, the trunnion is bent with the two spaced mount sides as fulcrum points. In particular, the middle portion of the trunnion between the two spaced mount sites moves inwardly into the traction cavity in one direction parallel to the axis of rotation of traction roller, while, the two remaining end portions move in the opposite direction. Such deviated moment of the trunnion from the pivot axis requires the corresponding movement of the feedback cam. The amount of such deviated movement varies with different magnitudes of input torque, causing varying of feedback information as to the same angular position of the trunnion with the different magnitudes of input torque.
A need remains for development of a ratio control wherein a feedback cam provides accurate feedback information over wide operating range of different magnitudes of input torque.
Various ratio control systems using a feedback cam have been proposed and assigned to the assignee of the present invention. One such example is shown in U.S. Pat. No. 5,885,185 issued Mar. 23, 1999 to Kidokoro et al. Another example is shown in U.S. Pat. No. 6,086,505 issued Jul. 11, 2000 to Sakai. Still another example is shown in U.S. Pat. No. 6,123,641 issued Sep. 26, 2000. Other example is shown in U.S. Patent Application Publication No. US2001/0046920 A1 published Nov. 29, 2001.
An object of the present invention is to provide a ratio control wherein a feedback cam provides accurate feedback information over wide operating range of different magnitudes of input torque.
According to one aspect of the present invention, there is provided a ratio control for a toroidal-type traction drive, comprising:
toroidal discs defining a toroidal cavity;
traction roller assemblies having pivot trunnions rotatably supporting traction rollers disposed in the toroidal cavity and engaged between the toroidal discs, each of the pivot trunnions having a pivot axis, the pivot trunnions including a predetermined pivot trunnion;
a ratio control element positionable to establish various ratios between the toroidal discs;
a feedback structure connected to the ratio control element and including a ratio control valve to hydraulically control the traction roller assemblies, and a cam follower;
a feedback cam cooperating with the cam follower; and
a lost motion cam actuator between the predetermined pivot trunnion and the feedback cam to permit limited independent deflected movement of the predetermined pivot trunnion from the pivot axis thereof without requiring corresponding movement of the feedback cam.